titter
Titter (verb) means to laugh quietly in a nervous or restrained way, often while trying not to.
Etymology
First recorded in the 1610s. It probably comes from Middle English *titeren, *titren (attested in the Middle English titering, meaning “hesitation, vacillation”), likely a frequentative of Middle English titten, meaning “to waver”, related to Old Norse titra, meaning “to shake, shiver, quiver”, and dialectal Swedish tittra, meaning “to snicker”.